Permanent magnet systems



` Feb. 13, 1962 B, w, MANLEY ETAL 3,021,458

PERMANENT MAGNET SYSTEMS Filed April 1o, 1959 2 sheets-sheet 1 7 FIG.1.

, AGENT Feb. 13, 1962 B. w. MANLEY ETAL 3,021,458

y PERMANENT MAGNET SYSTEMS Filed Aprily l0, 1959 2 Sheets-Sheet 2 FIG.3.

INVENTORS BRIAN w. MANLEY B BERNARD T. MURPHY United States Patent O rThe present invention relates to permanent magnet systems.

According to the present invention, Ia permanent magnet j system for focusing a beam of charged particles comprises a series of apertured pole-pieces, the aperture boundaries being figures of revolution about a common axis, and a series of magnets, the magnets and the pole-pieces being so associated that along the common axis is provided al field of substantially uniform ux the sense ofwhich is reversed in adjacent sections and the reversals being abrupt.

The system may comprise a pair of opposed magnets, each of which is a substantially cylindrical or a substantially frusto-conical shell of magnetic material, secured in position with the aid of supports providing an external ilux path for the magnets, the shells being spaced apar-t from one another along, and co-axial about, the common axis and each having pole-pieces at each end.

The pole-piece at one end of each magnet may extend inwardly of the magnet shell to form a co-axial inner shell and the pole-piece at the other end of said magnet towards and transversely of the common -axis into overlapping spaced relationship in the direction transverse to the common axis with the rst mentioned pole-piece.

Each pole-piece extending inwardly to form an inner shell may be secured to that end of the associated magnet shell which is remote from the air gap between the pair of magnets'.

The supports may be secured to extensions of the inwardly extending pole-pieces.

A coil may be provided for ne electro-magnetic adjustmen-t of the ux. The coil may be in one or a plurality of sections. y

The reversing flux provided by the system according to the present invention, may be used ito focus an electron beam, for example, in a klystron or a travelling-wave tube.

By dividing up the magnetic eld into the series of reversed sections less magnetic material isY required for a desired substantially uniform flux field. This is important for example where use in aircraft is envisaged.

In order that the present invention may be more readily understood and carried into effect, one embodiment thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which:

FIGURE l shows 4a part sectional view in elevation;

FIGURE 2 shows an external plan view; and

FIGURE 3 shows, on a reduced scale, a diagrammatic" view of a magnet system according to the present invention and showing the ux distribution (lwkage ux being neglected).

The system shown in FIGURE 1 comprises two permanent magnet shells 1, of substantially cylindrical shape and coaxially disposed with respect to each other, having pole-pieces 2 and 3 secured together by screws 4. The

` pole-pieces 3 have extensions 5 which are bored to receive connecting supports 6 which unite the two assemblies of shells and pole-pieces into a unit and which at the same time act as return paths for the llux. It will be seen that thepole-pieces 2 are inwardly directed towards a common axis passing through the shells in a direction substantially at right angles to the axis. l The pole-piece 5 is extended inwardly of the shell 1 so as to form an inner coaxial shell which is spaced from the pole-piece 2 so as to provide an air gap. From the above it will now be seen that there are three air gaps two of which are subsidiary gaps between the pole-pieces of each individual shell whereas the main gap extends between the two shells 1. Preferably the centre gap is twice the length of each of the subsidiary gaps.

In order to assist in obtaining the correct flux ow the innerk ends of the outer shells 1 are curved slightly inwardly as shown. In order to provide fine control of the flux strength, a coil is wound externally of the shells, this being connected, in operation, to an external source of current (not shown). The coil is shown divided into two parts 7 which may be connected in series.`

Ideally, the ux reversals are sharply defined. In practice, however, the abruptness of ythe reversals are limited in any particular case, by considerations of undesirable leakage ux which would -be produced by over-saturation of the pole-pieces. The pole-pieces 2 are shown as thinner towards the axis to provide a reasonably abrupt ux reversal with material sufficient to carry the ux towards the axis. FIGURE 3 shows the main flux ow and illustrates the opposing considerationsy discussed above.

The size of the central aperture is determined by the device with which it is intended the system is to be used and in any case the maximum size will be determined by the beam constants of the beam of charged particles produced in normal operationin the said device.

What we claim is:

1. A permanent magnet system for focussing a beam of charged particles, comprising, in order, four coaxial, axially-spaced -pole pieces having aligned central openings and defining between them three air-gaps, the portions of the pole pieces bounding their openings being symmetrically disposed about the axis, two spaced hollow permanent magnets both magnetized axially and in the same sense, one of said magnets being disposed between the first and second pole pieces and being magnetically coupled thereto, the other of said magnets being disposed between 3. A permanent magnet system for focussing a beam of charged particles, comprising, inorder, four coaxial, axially-spaced pole pieces having aligned central openings and defining between them three air-gaps, Ithe portions of the pole pieces bounding their openings being symmetrical-ly disposed about the axis, two spaced hollow, substantiall'y cylindrical permanent magnets both magnetized axially and in the same sense and coaxially aligned with the pole pieces, one of said magnets being disposed between and contacting the first and second pole pieces and surrounding the air-gap therebetween, the other of said magnets being disposed between and contacting `the third and fourth pole pieces and surrounding Ithe air-gap therebetween, and magnetic means outside yof the magnets and providing `a return ux path and maintaining the magnets and pole pieces in assembled relation, said system producing three regions along the axis in the air-gaps between the pole pieces of substantially uniform ux that is in opposite directions in adjacent regions.

4. A system as set forth in claim 3 wherein the maintaining magnetic means are secured to lateral extensions of the rst and fourth pole pieces, and means are provided for securing the magnets and second 'and third pole pieces to the first and fourth pole pieces.

5'. A system as set forth in claim 3 wherein the rst and fourth pole pieces each have `a cylindrical projection extending within the adjacent magnet toward the adjacent pole piece.

6. A system s set forth in claim 5 wherein the second and third pole pieces comprise plate-like members extending `transverse to the axis.

References Cited in the file of this patent UNITED STATES PATENTS 2,797,360 Rogers et a1. June 25, 1957 2,8'7aA-13r Reynst Feb. 10, 1959 2,940,020 Muller June 7, 1960 

